As part of ongoing projects to assess the human health risk posed by exposure to polyhalogenated hydrocarbons, peripheral blood lymphocytes were obtained from several cohorts of TCDD-exposed individuals. These samples were mitogen-stimulated in culture, alone or in the presence of TCDD, and the cells and culture supernatants harvested after 72 hr. The culture supernatants were evaluated for the presence of selected cytokines and for evidence of cytokine-induced isotype switching, as previous studies in our laboratory have suggested that this may be a consequence of TCDD exposure. To date we have examined these responses from a group of workers who were employed at a chemical plant in Hamburg-Moorfleet, Germany operated by Boehringer-Ingelheim from 1951-1984 that produced organochlorine herbicides and pesticides (including 2,4,5-trichlorophenoxyacetic acid) and opioids. TCDD contamination at this plant was confirmed in 1984 and significant increases in mortality have been observed in a cohort of workers employed at this plant for at least 3 months from 1952-1984. Peripheral blood for lymphocyte isolation and serum dioxin analysis was obtained from a cohort of 110 individuals in 1992, in collaboration with clinicians at the University of Mainz (Detlev Jung and Johannes Konietzko) and a biostatistician at the German Cancer Research Center (Lutz Elder). The secretion of the TH1 cytokines IFN-gamma and IL-2 and the TH2 cytokine IL-4 was significantly suppressed by TCDD treatment in vitro of lymphocytes from these workers. Levels of TNF-alpha were not significantly different in control and TCDD-treated cells. Cytokine secretion was not significantly associated with the level of in vivo dioxin exposure as measured by individual serum dioxin concentration, nor with other individual characteristics such as age or smoking. However, the data for IL-2 and TNFalpha suggest that the in vitro response to TCDD is weaker in individuals with high serum levels of dioxin. We also found that there are clear inter-individual differences in the magnitude of cytokine secretion. Ongoing analyses are focusing on determining whether the TCDD-induced down-regulation of the cytokine response in these individuals is associated with other well-characterized measures of individual response to TCDD such as induction of CYPA1 and CYP1B1 gene expression. These data suggest that alterations in TH1 cytokine secretion may be an important biomarker for TCDD exposure and may assist in predicting the susceptibility to immunologic alterations in exposed individuals.We have examined a second series of human lymphocytes culture supernatants obtained from subjects in the general population who were exposed to TCDD after an industrial explosion in 1976 in Seveso, Italy. These samples were obtained in collaboration with epidemiologists in the US (Neil Caporaso, NCI) and Italy (Pier Bertazzi and Maria Teresa Landi, University of Milan) and clinicians in Italy (Paolo Moccarelli, Desio Hospital in Seveso). The cohort under study consists of 120 individuals with equal numbers of exposed and non-exposed males and females. Serum 2,3,7,8-TCDD in the cohort ranges from non-detectable to 90 ppt lipid. Preliminary data indicate that secretion of IL-2 and IFNgamma is suppressed approximately 25% by in vitro TCDD treatment of lymphocytes in this cohort. IL-2 secretion did not appear to be correlated with CYP1A1 induction. The data will be analyzed in the same fashion as the Boehringer data and the two data sets will be compared as representative of chronic intermediate level (Seveso) and chronic high level (Boehringer) exposure.